Condensation bane

[MarkG]

hi all,

So here's my current learning challenge... condensation.

equipment:
Tripod half extended to keep COG low enough to reduce vibration distortion, but i'm on grass which helps too.
Skywatcher star adventurer 2i
Canon 550d + assorted lens

so i understand the technical principle of what happens to make condensation happen and i thought i could get around it by pre-acclimatizing the equipment to the environment but it didn't work.
so baring in mind this i a test of equipment, i was going for the andromeda galaxy with a 85-200 lens... not so much to get a great shot, but more about testing the star adventurer 2i and stacking some photographs to learn more about stacking and post processing with while away the nights beside a fire.

so the condensation strikes about an hour in to the shoot... 1st time around i set iso to 1600 20 shots as 2.5 mins each.. i got maybe 12 before i noticed condensation on lens.... not knowing about what to do.. i waited for pause in next shot and carefully wiped lens.... big no no i now know..

so second night... i put setup out in evening about 4 hours before i hope cloud cover would break, covered with light blanket and left it alone. same as before... iso 1600, 20 shots 2.5 min exposure... still getting condensation on lens.. so this time i called it quits and went and did some research...

Amazon my hero. i got a heat strap.. for about £14 delivered to me here in the boonies of Poland.

so that night, i set up again, attached heat strap, and battery... set the strap to low heat.. allowed to the heat up for an hour before aligning and prepping for first shots on the run...

condensation struck again... i repeated next night and upped the temp.. still got condensation.. went full high temp.. waited.. checked camera body and lens.. nice and warm which was a bit worrying... but started run.. still got condensation.

the best pick i got out of it is attached



so question is....how do you guys deal with condensation on lens and the like ?

clear skies!

mark

[Carole]

The image looks good.

Condensation, we all use dew heaters although that's not so easy with a Newtonian, though there are ways of doing it, but newtonians are less likley to dew up unless it is REALLY BAD. 

With a refractor normally the dew heater is placed around the telescope just slightly forward to the lens cell which is inside the tube.  The same can be done with a camera lens.

Dew heaters of course need power 12V and then a means of supplying that power, yes more purchases.

I use the W&W dew heaters, they are very good.  Turn your dew heaters on before you start, don't wait for the condensation to appear.

https://dewheater.com/

A hairdryer can get you out of an emergency, but no good for hours of imaging.

Carole

[ApophisAstros]

Quote
Carole
A hairdryer can get you out of an emergency, but no good for hours of imaging.

Unless you put that on a tripod..
Roger

[Mac]

Ahhh, the dreaded dew problems......

The reason why acclimatising doesn't work is your telescope cools down quicker than the ambient air.

At any temperature the air can only hold so much water

At a certain temperature (dew point) the amount of water that the atmosphere can hold is fully saturated  i.e 100% and that has to come out of saturation aka dew.
This constantly changes over the night as the temperature drops.

So when you start imaging it might be 20C and the dew point might be 14-15C
As the temperature drops and your telescope falls below that temperature the dew will form on its surfaces, Internally and externally.

To get around this  you need to heat the scope to above the dew point temperature (just the optics).
As Carole said a hairdryer is a temp quick fix,
Most of us use dew heaters, simple resistive networks connected to a PWM module and tapped to the telescope.
Mine used to have one around the main objective lens and one around the eyepiece, plus one on the camera. (All home made)

All you do is adjust the PWM module from 0 to 100% as the night draws in to keep your scope warm.

They are very easy to make and there are loads of circuit diagrams out there.

Mac.

[MarkG]

So a dew heater on a newtonian scope, does this not cause air vortices inside that might distort imaging? Especially at the edge where the tube is open to cold air. I would have though with a tube pointing up, cold air seeps down as hot air rises and causes distortions?

Mark

[The Thing]

Been there. It's the time of year.

1) Make sure the lenses are dry inside, you don't want condensation inside the lens - put in an airing cupboard for a day. If you have a vacuum storage box you can use that to remove moisture from 'sealed' units, easily available on ebay etc.

2) Knock up dew shields out of some cardboard and tape (absorbs a lot of the moisture). Check for vignetting, making it a bit cone shaped helps. The plastic camera ones are not long enough but can be used as a guide shape. The object is to shield the front element from space to prevent radiated heat loss as much as possible. E.G. the dew sheild for my SCT is ~1.5x the corrector plate diameter long (Macs' originally!)

3) Put the heater under the dew shield in line with the front element (keeps the heat in) (btw I used to sell dew heater kits) and turn it up until you can just feel its warm. You don't need much heat. You also don't need to wait as the point is to replace heat lost as IR from the front element to space, the lens surface just has to be kept a little warmer than ambient to stop the condensation.

Hope that helps

Duncan

[The Thing]

On my newt I have a homemade resistor dew heater around the outside of the OTA level with the primary mirror, I never have dew problems. I also have one around the finder guidescope as per my 2) and a dew shield on that made of old mouse mat and gaffa tape.

[garrick]

Hi Mark,

Dew straps are a must, but are you also using lens hoods on your Canon Lens.

Clear Skies,

G

[ApophisAstros]

i never needed either for my Canon 750d Modded as the chip produced enough heat to stop any Dew forming , although if you have cooling attached you would need something.
Rog..